How to change alpha for background image (camera) with python?
Python console
Given the tooltip, and the property being in 'DATA' panel can see that the property belongs to the data part of the camera.
I find autocomplete in the console handy for working these things out.
The data part of the scene camera (it's also the active object C.object.  In console C is a shortcut for bpy.context (see startup message).
&gt;&gt;&gt; C.scene.camera
bpy.data.objects['Camera']

&gt;&gt;&gt; cam = C.scene.camera.data
&gt;&gt;&gt; cam
bpy.data.cameras['Camera']

Ok, if you autocomplete on cam.&lt;TAB&gt; will notice that it has a background_images property...  Ok it's a collection
Here I've looped over the collection and displayed the alpha and its attached image.
&gt;&gt;&gt; for img in cam.background_images:
...     img.alpha, img.image
...     
(0.5, bpy.data.images['1b.jpg'])

Set first in list to alpha 1
&gt;&gt;&gt; cam.background_images[0].alpha = 1